The invention generally relates to thermal actuators of the buckling beam type, fabricated by microelectromechanical (MEM) technologies. Microelectromechanical (MEM) fabrication technologies, including surface micromachining methods based on integrated circuit (IC) manufacturing (e.g. semiconductor device manufacture), bulk micromachining, focused ion beam (FIB) processing, deep reactive ion etching (DRIE), LIGA (an acronym based on the first letters of the German words for lithography, electroplating and molding) and their combination, can be used to form microelectromechanical devices such as microsensors and microactuators, including buckling beam thermal actuators.
Dimensions of structures fabricated by MEM technologies can range from on the order of 0.1 μm, to on the order of a few millimeters, and include silicon, polysilicon, glass, dielectric and metallic structures that are either unsupported (i.e. free standing) or alternatively can be adhered to a substrate, or built up upon a substrate during manufacture. Substrates can comprise ceramics, glass-ceramics, low-temperature co-fireable ceramics (LTCC), quartz, glass, printed wiring boards (e.g. manufactured of polymeric materials including polytetrafluoroethylene, polyimide, epoxy, glass filled epoxy), silicon (e.g. silicon wafers), silicon on insulator (e.g. SOD substrates and metals. Dielectric layers for example, polymeric, silicon-oxide, silicon-nitride, glass and ceramic layers can be applied to the surface of conductive substrates (e.g. metallic and silicon substrates) to electrically isolate individual MEM elements within a fabricated structure, or isolate MEM elements from the substrate.
An exemplary surface micromachining technology is the Sandia Ultra-planar Multi-level MEMS Technology (SUMMiT™) available at Sandia National Laboratories, Albuquerque, N. Mex., wherein multiple polysilicon and dielectric layers are used to form mechanical structures on a silicon substrate, as described in the commonly owned patents, U.S. Pat. No. 5,804,084 to R. Nasby et al., and U.S. Pat. No. 6,082,208 to M. Rodgers et al., the entirety of their disclosures herein incorporated by reference. Additionally as described in the design guide “SUMMiT V™, Five level Surface Micromachining Technology Design Manual”, Version 3.0, Jan. 18, 2007, [online] [retrieved on Jan. 14, 2008] retrieved from the Internet: <URL:http://www.mems.sandia.gov/sample/doc/SUMMiT_V_Dmanual_V3.0.pdf>, the entirety of the disclosure incorporated herein by reference, structural elements (e.g. buckling beams) can be fabricated utilizing up to five layers (or combinations thereof) of patterned polysilicon with the individual polysilicon layers ranging in thickness from approximately 0.3 μm up to approximately 2.25 μm, and dielectric layers comprising silicon oxides and silicon nitride layers ranging from approximately 0.63 μm up to approximately 2.0 μm per dielectric layer.
Structural elements formed from layers that are thicker than typically available in a multi-level polysilicon technology, can comprise single crystal silicon structural elements fabricated using silicon on insulator (SOI) substrates and surface micromachining methods as described in commonly owned patents, U.S. Pat. No. 7,289,009 to T. Christenson et al., and U.S. Pat. No. 7,038,150 to M. Polosky et al., the entirety of their disclosures herein incorporated by reference. SOI substrates can comprise a base layer of up to approximately 500 μm of single crystalline silicon, and a dielectric layer of up to 200 μm silicon oxide (e.g. SiO2) insulating the base layer of silicon from a second layer of single crystal silicon that can be up to approximately 500 μm thick. Thicker structural elements, as for example incorporated into a buckling beam thermal actuator, can provide correspondingly greater actuation forces. Within the context of this disclosure, silicon oxide refers to oxides of silicon that may either be thermally grown or deposited by chemical vapor deposition methods and can comprise the stoichiometric composition (SiO2) as well as non-stoichiometric compositions (SiOx).
MEM buckling beam thermal actuators generally comprise an elongated member, for example a beam formed by patterning one or more layers of polysilicon or a layer of single crystal silicon, which is rigidly attached to a substrate, for example a silicon base, by dielectric supports at each end of the beam. Heating the beam, for example by passing an electrical current through it, causes the beam to expand and eventually buckle. The force generated by the buckling motion of the beam, generally in a direction perpendicular to its length, can be harnessed to perform useful work as an actuator. An issue for buckling beam thermal actuators is controlling (i.e. constraining) the buckling direction of the beam to be in the desired direction of actuation for the actuator. For example, initially straight beams of uniform cross-section along their length can buckle in more than one direction, and measures need be undertaken to insure that such a beam buckles, i.e. provides actuation, in the direction desired.
Methods to control the actuation direction of MEM thermal actuators include forming the elongated element in an initial shape such as a “V-beam”, a chevron shape or a curved shape, producing a beam with variable cross-sections, and coupling an array of beams of differing cross-sections. These approaches can add complexity to the processes used to fabricate thermal actuators. Embodiments of the present invention overcome these difficulties by incorporating a directional constraint element which acts as a mechanical stop and/or limiter, to ensure buckling occurs in the desired direction of actuation.